Properties Of Two-dimensional Electron Gas Research Articles

Magnetothermal properties of two-dimensional electron gas in matched AlGaAs/GaABiN structure

Magnetothermal properties of two-dimensional electron gas in matched AlGaAs/GaABiN structure

Gate-controlled hysteresis curves and dual-channel conductivity in an undoped Si/SiGe 2DEG structure

Exploring the cryogenic transport properties of two-dimensional electron gas in semiconductor heterostructures is always a focus of fundamental research on Si-based gate-controlled quantum devices. In this work, based on the electrical and magnetic transport characteristics of Si/SiGe heterostructure Hall bar-shaped field effect transistors (FETs) at 10 K and 1.6 K, we study the effects of electron tunneling, which occurs in the heterostructure and populates the oxide/semiconductor interface, on its transport properties. The initial position of dual-channel conduction is determined by the gate-controlled electrical hysteresis curves. Furthermore, we discover that there exist different tunneling mechanisms of electrons in a Si quantum well under the action of gate voltage, and the electron tunneling can well explain the two drain current plateaus appearing in direct-current transfer characteristics. Combining the power-law exponent of electron mobility versus density curve and the gate-related Dingle ratio, we clarify the dominant scattering mechanism, and the result can be supported by different tunneling mechanisms. Our work demonstrates the gate-dependent electronic transport performance in undoped Si/SiGe heterostructure FETs, which has an implication for the development of Si/SiGe heterostructure gate-defined quantum dot quantum computation.

Status and Prospects of Heterojunction-Based HEMT for Next-Generation Biosensors

High electron mobility transistor (HEMT) biosensors hold great potential for realizing label-free, real-time, and direct detection. Owing to their unique properties of two-dimensional electron gas (2DEG), HEMT biosensors have the ability to amplify current changes pertinent to potential changes with the introduction of any biomolecules, making them highly surface charge sensitive. This review discusses the recent advances in the use of AlGaN/GaN and AlGaAs/GaAs HEMT as biosensors in the context of different gate architectures. We describe the fundamental mechanisms underlying their operational functions, giving insight into crucial experiments as well as the necessary analysis and validation of data. Surface functionalization and biorecognition integrated into the HEMT gate structures, including self-assembly strategies, are also presented in this review, with relevant and promising applications discussed for ultra-sensitive biosensors. Obstacles and opportunities for possible optimization are also surveyed. Conclusively, future prospects for further development and applications are discussed. This review is instructive for researchers who are new to this field as well as being informative for those who work in related fields.

Scattering Analysis of AlGaN/AlN/GaN Heterostructures with Fe-Doped GaN Buffer.

The results of the study of the influence of Fe segregation into the unintentionally doped GaN channel layer in AlGaN/AlN/GaN heterostructures with Fe-doped GaN buffer layer on the electrical properties of two-dimensional electron gas are presented. A set of several samples was grown by metal-organic vapor-phase epitaxy and characterized by the van der Pauw method. The dependence of concentration and mobility of the two-dimensional electron gas on the channel layer thickness was analyzed theoretically by self-consistent solving of 1D Poisson and Schrödinger equations and scattering rate calculations within the momentum relaxation time approximation. It was found that both concentration and mobility decreases were responsible for the increase in the sheet resistance in the structures with a thinner channel layer, with a drop in mobility being not only due to ionized impurity scattering, but also due to a combined effect of weakening of screening, lower carrier energy and change in form-factors on scattering by interface roughness, dislocations and polar optical phonons.

Scattering suppression at MOS interface towards high-mobility Si-based field-effect transistors

The severe scattering at metal-oxide-semiconductor (MOS) interfaces, manifesting as carrier mobility declining, is always a puzzle in the field of Si-based field-effect transistors (FETs) towards high-performance devices. In this work, an elaborate study on interfacial scattering suppression through process optimization is reported. Dry oxidation is proved to be more efficient in the construction of a damage-free oxide/semiconductor interface. Meanwhile, a thick thermal-SiO2 interlayer (i.e. a long growth time), together with a long-duration post-annealing treatment, is beneficial for interface planarization. Such an interface roughness scattering control, combined with Coulomb and defect scattering restrictions, can be realized with an optimized FET process flow. On this occasion, the as-fabricated Si MOSFETs show a higher gate-controlled drain current, and are with a peak electron mobility of ∼8372 cm2 V−1 s−1 at 1.6 K. Moreover, confined magnetotransport properties of two-dimensional electron gas are further revealed by the integer quantum Hall effects. Notably, our work presents a feasible integration flow to fabricate high-mobility Si MOS devices via scattering suppression, which may promote the evolution of Si-based MOS quantum dots for potential solid-state quantum computing.

Influence of AlN/GaN interfacial non-idealities on the properties of two-dimensional electron gas in AlGaN/AlN/GaN heterostructures

The influence of two types of AlN/GaN interfacial non-idealities, namely unintentional Ga incorporation into AlN spacer and blurring of the spacer due to Al and/or Ga atomic diffusion on the mobility and density of two-dimensional electron gas in AlGaN/AlN/GaN heterostructure was studied theoretically. It was found that moderate amount of GaN in the nominal AlN spacer does not affect much the mobility and density of 2DEG as long as the interface is abrupt. In contrast, the blurring of AlN/GaN interface was found to have a significant impact on the mobility and sheet resistance of the structure since the GaN channel actually becomes AlGaN and alloy-disorder scattering takes place.

Electron effective masses, nonparabolicity and scattering times in one side delta-doped PHEMT AlGaAs/InGaAs/GaAs quantum wells at high electron density limit

The dependence of electron transport properties of two-dimensional electron gas on sheet doping concentration in one-sided δ-doped pseudomorphic AlxGa1-xAs/In0.2Ga0.8As/GaAs quantum wells is investigated. The wide range of silicon dopant sheet concentrations of (1.6–16) · 1012 cm−2 is investigated. Electron effective masses, nonparabolicity and scattering times are determined by low-temperature Shubnikov-de Haas effect measurements. The dependence of the quantum and transport relaxation times on nH is shown to have nonmonotonic character due to the competition of the Fermi momentum increase and the large angle scattering due to the variation of ionized donor concentration. The nonparabolicity coefficient in the In0.2Ga0.8As quantum well is determined to be 0.68 1/eV.

Distinguishing various influences on the electrical properties of thin-barrier AlGaN/GaN heterojunctions with in-situ SiNx caps

Understanding the individual contributions of the factors that influence the electrical properties of a material is important for controlling these properties. In this study, the effects of multiple factors on the electrical properties of thin-barrier AlGaN/GaN heterojunctions with in-situ SiNx caps were investigated using step etching and annealing treatments. The sheet density of the two-dimensional electron gas decreased by 5.9% and 29.5% due to the decrease in the piezoelectric polarization of the AlGaN barrier and the variations in interface charges during SiNx removal, respectively, and it recovered greatly by 72% after annealing-induced surface reconstruction. Capacitance-voltage results clearly revealed few interface traps on the heterojunction with the in-situ SiNx cap. Moreover, a maximum output current of 705 mA/mm and threshold voltage hysteresis below 50 mV were achieved by optimizing the in-situ SiNx interlayer on a thin-barrier metal-insulator-semiconductor structure. This study presents an efficient method for modulating the properties of two-dimensional electron gas and provides insights into the functionality of in-situ SiNx passivation on thin-barrier AlGaN/GaN heterojunctions.

Electric gating of the multichannel conduction in LaAlO3/SrTiO3 superlattices* *Project supported by the National Basic Research Program of China (Grant Nos. 2016YFA0300701, 2017YFA0206300, 2017YFA0303601, and 2018YFA0305704), the National Natural Science Foundation of China (Grant Nos. 11520101002, 51590880, 11674378, 11934016, and 51972335), and the Key Program of the Chinese Academy of Sciences.

The electric gating on the transport properties of two-dimensional electron gas (2DEG) at the interface of LaAlO3/SrTiO3 (LAO/STO) heterostructure has attracted great research interest due to its potential application in field-effect devices. Most of previous works of gate effect were focused on the LAO/STO heterostructure containing only one conductive interface. Here, we systematically investigated the gate effect on high-quality LAO/STO superlattices (SLs) fabricated on the TiO2-terminated (001) STO substrates. In addition to the good metallicity of all SLs, we found that there are two types of charge carriers, the majority carriers and the minority carriers, coexisting in the SLs. The sheet resistance of the SLs with a fixed thickness of the LAO layer increases monotonically as the thickness of the STO layer increases. This is derived from the dependence of the minority carrier density on the thickness of STO. Unlike the LAO/STO heterostructure in which minority and majority carriers are simultaneously modulated by the gate effect, the minority carriers in the SLs can be tuned more significantly by the electric gating while the density of majority carriers is almost invariable. Thus, we consider that the minority carriers may mainly exist in the first interface near the STO substrate that is more sensitive to the back-gate voltage, and the majority carriers exist in the post-deposited STO layers. The SL structure provides the space separation for the multichannel conduction in the 2DEG, which opens an avenue for the design of field-effect devices based on LAO/STO heterostructure.

Photoinduced electronic and spin properties of two-dimensional electron gases with Rashba spin-orbit coupling under perpendicular magnetic fields

We theoretically investigate photoinduced phenomena induced by time-periodic driving fields in two-dimensional electron gases under perpendicular magnetic fields with Rashba spin-orbit coupling. Using perturbation theory, we provide analytical results for the Floquet-Landau energy spectrum appearing due to THz radiation. By employing the resulting photo-modulated states, we compute the dynamical evolution of the spin polarization function for an initially prepared coherent state. We find that the interplay of the magnetic field, Rashba spin-orbit interaction and THz radiation can lead to inversion of the spin polarization. The dynamics also induces fractional revivals and non-trivial beating patterns in the autocorrelation function due to interference of the photo-modulated quantum states. We also calculate the transverse photo-assisted conductivity in the linear response regime using Kubo formalism and analyze the impact of the radiation field and Rashba spin-orbit interaction. In the static limit, we find that our results reduce to well-known expressions of the conductivity in non-relativistic and quasi-relativistic (topological insulator surfaces) two-dimensional electron gas thoroughly described in the literature. We discuss the possible experimental detection of our theoretical prediction and their relevance for spin-orbit physics at high magnetic fields.

Experimental measurements of effective mass in near-surface InAs quantum wells

Near surface indium arsenide quantum wells have recently attracted a great deal of interest since they can be interfaced epitaxially with superconducting films and have proven to be a robust platform for exploring mesoscopic and topological superconductivity. In this work, we present magnetotransport properties of two-dimensional electron gases confined to an indium arsenide quantum well near the surface. The electron mass extracted from the envelope of the Shubnikov-de Haas oscillations shows an average effective mass $m^{*}$ = 0.04 at low magnetic field. Complementary to our magnetotransport study, we employed cyclotron resonance measurements and extracted the electron effective mass in the ultra high magnetic field regime. Our measurements show that the effective mass depends on magnetic field in this regime. The data can be understood by considering a model that includes non-parabolicity of the indium arsenide conduction bands.

Electronic and optical properties of monolayer tin diselenide: The effect of doping, magnetic field, and defects

A parameterized tight-binding (TB) model based on the first-principles GW calculations is developed for single layer tin diselenide (SnSe$_2$) and used to study its electronic and optical properties under external magnetic field. The truncated model is derived from six maximally localized wannier orbitals on Se site, which accurately describes the quasi-particle electronic states of single layer SnSe$_2$ in a wide energy range. The quasi-particle electronic states are dominated by the hoppings between nearest wannier orbitals ($t_1$-$t_6$). Our numerical calculation shows that, due to the electron-hole asymmetry, two sets of Landau Level spectrum are obtained when a perpendicular magnetic field is applied. The Landau Level spectrum follows linear dependence on the level index and magnetic field, exhibiting properties of two-dimensional electron gas in traditional semiconductors. The optical conductivity calculation shows that the optical gap is very close to the GW value, and can be tuned by external magnetic field. Our proposed TB model can be used for further exploring the electronic, optical, and transport properties of SnSe$_2$, especially in the presence of external magnetic fields.

Electron mobility enhancement in an undoped Si/SiGe heterostructure by remote carrier screening

We investigate the effects of surface tunneling on electrostatics and transport properties of two-dimensional electron gases (2DEGs) in undoped Si/SiGe heterostructures with different 2DEG depths. By varying the gate voltage, four stages of density-mobility dependence are identified with two density saturation regimes observed, which confirms that the system transitions between equilibrium and nonequilibrium. Mobility is enhanced with an increasing density at low biases and, counterintuitively, with a decreasing density at high biases as well. The density saturation and mobility enhancement can be semiquantitatively explained by a surface tunneling model in combination with a bilayer screening theory.

Quantum Transport Properties of Two-Dimensional Quantum Lattices under Synthetic Magnetic Fields

Motivated by recent experimental progress, we study the quantum transport properties of two-dimensional electron gases under high perpendicular magnetic fields. We use a simple tight-binding model to model the system and open-source software to simulate quantum electronic transport properties such as band structure variations and conductance-flux relationships in such systems. Dependence of quantum transport properties on two-dimensional square, triangular and kagome lattice shapes were studied adding a Gaussian noise to account for the impurities. Numerical simulations are presented to predict the emergence of physical effects related to quantum Hall effect, such as the existence of Landau levels and edge states. The kagome lattice exhibits a different band structure giving rise to a flat band, due to its trihexagonal geometry. The peak conductance value increases with decreasing lattice constant due to higher transmission probability. The transport properties vary significantly with lattice geometries, both with the lattice type and the lattice constant.

Quantum Transport Properties of Two-Dimensional Quantum Lattices under Synthetic Magnetic

Motivated by recent experimental progress, we study the quantum transport properties of two-dimensional electron gases under high perpendicular magnetic fields. We use a simple tight-binding model to model the system and open-source software to simulate quantum electronic transport properties such as band structure variations and conductance-flux relationships in such systems. Dependence of quantum transport properties on two-dimensional square, triangular and kagome lattice shapes were studied adding a Gaussian noise to account for the impurities. Numerical simulations are presented to predict the emergence of physical effects related to quantum Hall effect, such as the existence of Landau levels and edge states. The kagome lattice exhibits a different band structure giving rise to a flat band, due to its trihexagonal geometry. The peak conductance value increases with decreasing lattice constant due to higher transmission probability. The transport properties vary significantly with lattice geometries, both with the lattice type and the lattice constant.

AlGaN/GaN heterostructures with an AlGaN layer grown directly on reactive-ion-etched GaN showing a high electron mobility (>1300 cm2 V−1 s−1)

In this study, the metal–organic-vapor-phase-epitaxial growth behavior and electrical properties of AlGaN/GaN structures prepared by the growth of an AlGaN layer on a reactive-ion-etched (RIE) GaN surface without regrown GaN layers were investigated. The annealing of RIE-GaN surfaces in NH3 + H2 atmosphere, employed immediately before AlGaN growth, was a key process in obtaining a clean GaN surface for AlGaN growth, that is, in obtaining an electron mobility as high as 1350 cm2 V−1 s−1 in a fabricated AlGaN/RIE-GaN structure. High-electron-mobility transistors (HEMTs) were successfully fabricated with AlGaN/RIE-GaN wafers. With decreasing density of dotlike defects observed on the surfaces of AlGaN/RIE-GaN wafers, both two-dimensional electron gas properties of AlGaN/RIE-GaN structures and DC characteristics of HEMTs were markedly improved. Since dotlike defect density was markedly dependent on RIE lot, rather than on growth lot, surface contaminations of GaN during RIE were believed to be responsible for the formation of dotlike defects and, therefore, for the inferior electrical properties.

Advanced Terahertz Frequency-Domain Ellipsometry Instrumentation for In Situ and Ex Situ Applications

We present a terahertz (THz) frequency-domain spectroscopic ellipsometer design that suppresses formation of standing waves by use of stealth technology approaches. The strategy to suppress standing waves consists of three elements geometry , coating , and modulation . The instrument is based on the rotating analyzer ellipsometer principle and can incorporate various sample compartments, such as a superconducting magnet, in situ gas cells, or resonant sample cavities, for example. A backward wave oscillator and three detectors are employed, which permit operation in the spectral range of 0.1–1 THz (3.3–33 cm $^{-1}$ or 0.4–4 meV). The THz frequency-domain ellipsometer allows for standard and generalized ellipsometry at variable angles of incidence in both reflection and transmission configurations. The methods used to suppress standing waves and strategies for an accurate frequency calibration are presented. Experimental results from dielectric constant determination in anisotropic materials, and free charge carrier determination in optical Hall effect (OHE), resonant-cavity enhanced OHE, and in situ OHE experiments are discussed. Examples include silicon and sapphire optical constants, free charge carrier properties of two-dimensional electron gas in a group III nitride high electron mobility transistor structure, and ambient effects on free electron mobility and density in epitaxial graphene.

P-GaN Gate Enhancement-Mode HEMT Through a High Tolerance Self-Terminated Etching Process

An enhancement-mode high-electron-mobility transistor with a p-GaN gate was fabricated by using a chemistry-ease Cl2/N2/O2-based inductively coupled plasma etching technique. This etching technique features a precise etching self-termination at the AlGaN barrier surface, which enables a broad process window with a large tolerance of etching time. With a post-annealing process, the property of two-dimensional electron gas (2DEG) can be restored to a high level after the etching. The mechanisms of etching self-termination and 2DEG recovery were clarified. The fabricated device exhibits a drain saturation current of 355 mA/mm with a threshold voltage of +1.1 V, an on/off ratio of $10^{7}$ , and a static on-resistance ${R} _{\mathrm{ ON}}$ of $10~{\Omega }\cdot $ mm. Furthermore, normally-off operation of the device can be achieved across the wafer.

Dirac fermions and pseudomagnetic fields in two-dimensional electron gases with triangular antidot lattices

We investigate theoretically the electronic properties of two-dimensional electron gases (2DEGs) with regular and distorted triangular antidot lattices. We show that the triangular antidot lattices embedded in 2DEGs behave like artificial graphene and host Dirac fermions. By introducing the Wannier representation, we obtain a tight-binding Hamiltonian including the second-nearest-neighboring hopping, which agrees well with the numerically exact solutions. Based on the tight-binding model, we find that spatially nonuniform distortions of the antidot lattices strongly modify the electronic structures, generate pseudomagnetic fields and the well-defined Landau levels. In contrast to graphene, we can design the nonuniform distortions to generate various configurations of pseudomagnetic fields. We show that the snake orbital states arise by designing the $\ifmmode\pm\else\textpm\fi{}\mathbf{B}$ pseudomagnetic field configuration. We find that the disorders of antidot lattices during fabrication would not affect the basic feature of the Dirac electrons, but they lead to a reduction in conductance in strong disorder cases.

Multi-photon transitions in coupled plasmon-cyclotron resonance measured by millimeter-wave reflection

We construct a low-temperature microwave waveguide interferometer for measuring the high-frequency properties of two-dimensional electron gases. Coupled plasmon-cyclotron resonance (PCR) spectra are used to extract effective mass, bulk plasmon frequency, and carrier relaxation times. In contrast to traditional transmission spectroscopy, this method does not require sample preparation and is nondestructive. PCR signals can be resolved with a microwave power source as low as 10 nW. We observe PCR in the multi-photon transition regime, which has been proposed to be relevant to the microwave-induced resistance oscillations.